hdu2157：How many ways??

Posted 2020-10-06 Blue233333

n<=20个点m<=100条边有向图不带权，t个询问问Ai到Bi的经过k<=20条边方案数多少。

f[i][j]--i到j的方案数，，初始化成初邻接矩阵，这样做一次就得到2条路最短路，做两次就是4条……这东西不就是矩乘吗？

 1 #include<stdio.h>
 2 #include<string.h>
 3 #include<stdlib.h>
 4 #include<algorithm>
 5 //#include<iostream>
 6 using namespace std;
 7 
 8 int n,m,t;
 9 #define maxn 23
10 typedef int mat[maxn][maxn];
11 mat a,ans;
12 const int mod=1000;
13 void copy(mat a,mat b)
14 {
15     for (int i=1;i<=n;i++)
16         for (int j=1;j<=n;j++)
17             a[i][j]=b[i][j];
18 }
19 void mul(mat a,mat b,mat ans)
20 {
21     mat t;memset(t,0,sizeof(t));
22     for (int i=1;i<=n;i++)
23         for (int j=1;j<=n;j++)
24             for (int k=1;k<=n;k++)
25                 t[i][j]=(t[i][j]+a[i][k]*b[k][j]%mod)%mod;
26     copy(ans,t);
27 }
28 void init(mat t)
29 {
30     memset(t,0,sizeof(mat));
31     for (int i=1;i<=n;i++) t[i][i]=1;
32 }
33 void pow(mat a,int b,mat ans)
34 {
35     mat t,tmp;init(t);copy(tmp,a);
36     while (b)
37     {
38         if (b&1) mul(t,tmp,t);
39         mul(tmp,tmp,tmp);
40         b>>=1;
41     }
42     copy(ans,t);
43 }
44 int main()
45 {
46     while (scanf("%d%d",&n,&m),n||m)
47     {
48         int x,y,v;memset(a,0,sizeof(a));
49         for (int i=1;i<=m;i++)
50         {
51             scanf("%d%d",&x,&y);
52             x++;y++;
53             a[x][y]=1;
54         }
55         scanf("%d",&t);
56         for (int i=1;i<=t;i++)
57         {
58             scanf("%d%d%d",&x,&y,&v);
59             x++;y++;
60             pow(a,v,ans);
61             printf("%d\\n",ans[x][y]);
62         }
63     }
64     return 0;
65 }

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